Signaling system



May 3, 1949. H. MONTGOMERY 2,458,832

f SIGNALING SYSTEM l Filed Sept. 9, 1947 2 Sheets-Sheet 1 WIW l fw@ ff lfm? f5 7 ze la /F/f/e/ W/THOUT CONT/POL WITH 60A/WOL ATTORNEYS 2 Sheets-Sheet 2 OUTPUT L 76 l I Il Il l l l I l l ll l I .l l

Patented May 3, 1949 UNITED STATES PATENT OFFICE .SIGNALING SYSTEM Luke H. Montgomery, Nashville, Tenn.

-Application September 9, 1947 :Serial No, 772,912

Claims. 'l

This invention relates to signaling circuits and provides a method and .apparatus for controlling the operation of such circuits in relation to the waveform symmetry of an alternating signal wave. While not confined thereto in its broader aspects, the invention is particularly directed to audio-frequency signal :modulation circuits in which audio signals occasionally are asymmetric.

In radio transmitting stations the audio signal waves used to modulate the carrier are usually symmetrical in their positive and negative amplitude excursions. Transmitter circuits are com.- monly designed so that the loudest signals to be transmitted will produce close to 100% modulation, for maximum efficiency. It has been found that certain voices give asymmetrical audio waves which, in .extreme cases, may give only modulation in one direction for a signal level giving 100% modulation in the .opposite direction. The asymmetry may be in either direction, that is, the positive amplitudes 4may be greater than the negative, or vice-versa, depending upon the voice. Likewise, certain musical instruments upon occasion appear to yield asymmetrical waves. Although the asymmetry is believed due to peculiarities in the voices or instruments, certain types of microphones seem more prone to give asymmetrical responses for certain voices or instruments than other types.

If a transmitter is adjusted to give 100% modulation for symmetrical audio signals of given volume, asymmetrical signals `of the same volume will result in overmodulating the transmitter in one direction and under-modulating it in the other. Over-modulation results in distortion, and under-modulation results in failure to transmit the signal at the required volume level. To avoid distortion due to over-modulation, it is customary to reduce .the amplitude of the audio signal wave.

The present invention prevents over-modulalation by asymmetrical .signals without reducing the amplitude thereof, thus permitting transmission at full volume level without distortion. This is accomplished, broadly, by developing a control wave which isa function of the asymmetry of the signal wave, and then utilizing the control wave to shift the normal unmodulated operating point of the modulation circuit.

Modulation may be dened as the process of modifying a carrier, some characteristic of which is made to vary as a function of the instantaneous value of a modulating signal wave. Amplitudemodulation and Ifrequency-modulation are the principal types used in broadcasting at present.

In amplitude-modulation the amplitude characteristic ofthe carrier is varied in accordance with the signal wave. In frequency-modulation the frequency characteristic of the carrier is varied. The present invention may be applied broadly to any type of modulation, but is particularly useful in connection with amplitude and frequency modulation.

In the specific embodiments -described hereinafter., a control wave is produced by applying the signal wave to a rectier or detector circuit which yields an -output which varies with the degree .of asymmetry of positive and negative portions oi the signal. It is particularly advantageous to utilize peak .detector Acircuits so as to obtain a control wave which varies with differences in the peak positive and negative amplitudes of the signal wave. The control Wave is then supplied to the modulation circuit to shift the normal unmodulated carrier value when the signal wave is asymmetrical. .l

The invention will be more clearly understood by reference to the drawings and the following description thereof. In the drawings:

Fig. 1 is a diagram showing the 4manner of deriving the asymmetry control wave and applying it to a modulation circuit;

Fig. 2 is a diagram showing the effect 'of the asymmetry control in amplitude modulation;

Fig. 3 is a specific embodiment of an -amplitudemodulatio-n circuit utilizing a control circuit of the invention;

Fig. 4 is .a diagram of a frequency-modulation circuit utilizing a control circuit oi the invention; and

Fig. 5 is .a diagram showing the veffect of the asymmetry control in `fr-euiuency modulation.

Referring to Fig. l, a modulator and modulated amplier Il is supplied with a radio-frequency carrier input l2 and an audio signal input i3. The audio signal input I3 is .also supplied to an asymmetry control circuit, generally designated as It, through a buler amplier I5. Transformer IB is fed from amplifier l5 and has a center tapped secondary. A Apair of rectiiiers ll and AI8 are connected in series with respective impedances 2| and 22 between the center .tap and respective ends of the secondary of transformer -l B. The rectiflers are polarized so that the waves built up in impedances 21 and 22 are'in opDOSi-tion. The resulting control` wave across terminals 23 is therefor the difference between the lWaves built up in impedances 2l and 22, These impedances are reactive so as to provide a fairly smooth output control wave.

The control wave is then supplied to the modulator and modulated amplifier Il so as to change the normal unmodulated value in accordance with the asymmetry of the signal input wave. The output of Il is supplied through terminals 24 to the radio-frequency transmitting circuit.

In Fig. 2, a symmetrical signal wave is shown at A, an asymmetrical signal wave with greater negative excursion at B, and an asymmetrical wave with greater positive excursion at C. Below these waves are corresponding amplitude-modulated radio-frequency waves D, E and F. The dotted lines PC represent the peak carrier amplitude for 100% modulation. The dotted lines UC represent the normal unmodulated carrier level.

It will be observed that wave D shows practically l% modulation, with the carrier amplitude varying from nearly zero to peak amplitude. Asymmetrical wave B has the same peak-to-peak amplitude as wave A, yet the positive amplitude excursion in E is not as great as in D so that the carrier is not as fully modulated. The negative excursion results in over-modulating the carrier with consequent distortion. In F, the positive peak of wave C e'oes beyond the peak carrier amplitude and this over-modulation results in distortion. On the other hand, the negative eX- cursion does not reduce the carrier level as much as in D.

The lower row of Waves, G, H and I, represent radio-frequency amplitude-modulated Waves with the asymmetry control of the invention. The normal symmetrical modulation shown in G is the same as in D. In H, however, the normal unmodulated carrier level has been increased to 3|. Thus the asymmetrical wave B modulates the carrier fu-lly from zero to peak amplitude levels, without over-modulation or distortion. In I, the control wave reduces the normal unmodulated carrier level to 32, so that the modulating wave shown at C again fully modulates the carrier from nearly zero to peak amplitude without over-modulation or distortion.

Fig. 3 shows one specific embodiment of applicants invention used with a conventional plate, or'I-Ieising type modulation circuit. Here the audio signal input wave is supplied through audio amplifier 33 to the control grid -of modulator tube 34. A battery 40 supplies suitable grid bias. Plate voltage for tube 34 is supplied from a B+ source through coupling inductance 35. The plate of tube 34 is connected through R. F. choke 36 to the plate of the modulated-amplifier tube 31. The radio-frequency carrier is supplied to the primary of transformer 33, and the secondary is shunted by capacitor 39 tuned to resonance. The carrier is then supplied through grid leak resistor 4I and shunting capacitor 42 to the grid 43 of the modulated amplifier tube 31. The output circuit of tube 31 includes the tank circuit 44 and blocking capacitor 45.

The plate supply voltage for the modulated -amplier 31 is controlled by electronic tube 46 connected between cathode 41 and ground. Tube 418 is shunted by capacitor 48 to bypass the radio frequency component. It will be understood that as the potential applied to grid 5i of tube 46 1S varied, the internal resistance of the tube varies and hence the plate supply voltage of tube 31 may be altered. Thus the carrier level may be shifted by changing the grid potential of tube 4B.

The control circuit in Fig. 3 is similar to that shown in Fig. 1. The audio signal input is supplied from amplier 33 to transformer I S, and rectiiiers l1 and I8 are connected in the secondary circuit of the transformer. The impedances are here shown as capacitors 52 and 53 shunted by respective resistors 54 and 55. Each of rectiers I1 and I8 serves as a peak detector, in that the Voltage built up across capacitor 52 represents the peak amplitude of 'one polarity of the wave, and the voltage built up across capacitor 53 represents the peak amplitude of the other polarity. Since capacitors 52 and 53 are in series between ground and grid 5I, the voltages are in opposition and the potential applied to grid 5l varies in accordance with the difference between the posi- Itive and negative peak voltages. The sign of the combined voltage across capacitors 52 and 53 is determined by the asymmetrical peak of greater amplitude. A battery 51 may be provided to establish proper operating grid bias for tube 4E.

When the audio signal wave is symmetrical, no voltage is developed across capacitors 52 and 53, and the bias applied to the grid of tube 45 remains xed at a Value determined by battery 51. Thus the plate voltage of tube 31 is not varied by tube 46 and the circuit operates in the normal fashion as a plate modulation circuit. However, when the audio signal wave is asymmetrical, a voltage of proper polarity is developed across capacitors 52 and 53 and changes the grid `bias on grid tube 4E. This results in changing the plate voltage across tube 31 and the normal unmodulated carrier level is shifted.

It will therefore be clear that the normal operating point on the transfer characteristic between audio signal input and modulated carrier output is shifted in proportion to the magnitude and polarity of the control voltage wave, which in turn are determined by the degree and direction of asymmetry of the audio signal wave.

The polarity of the control voltage is selected so that the shift in carrier level is in the direction of the modulation produced by the smaller of the asymmetrical amplitudes. For example, referring to Fig. 2, the positive portions of wave B are of smaller amplitude than the negative portion, and the shift in the unmodulated carrier level shown at 3| in H is in the direction of the modulation produced by the positive `portion of `the modulating wave.

The circuit constants of the control circuit are selected in accordance with the constants of the modulation circuit and the conditions of operation. Rectiers I1 and I8 may be of any suitable type. For example, crystal rectiers or detectors of type 11135 hav-e been employed with success, but diodes may be employed if desired. It is advantageous to select values for capacitors 52 and 53 which will permit their rapid charge to peak voltage, so that the attack time is kept short. The values of shunting resistors 54 and 55 are advantageously selected so that the control wave is smoothed for the lowest audio frequency it is desired to transmit. In some cases the back resistance of crystal rectiiier may provide sumcient leakage to permit dispensing with separate resistors. Time constants of the order of onetenth of a second have been employed with success. In this manner any very low frequencies which may be injected into the modulated signal through the control circuit lare kept below the frequency response of the receivers which utilize the transmitted signal.

It will be understood that as soon as the signal wave returns to its normal symmetrical condition, the total control voltage across capacitors 52 and 53 will be rapidly returned to zero. In the event that momentary over-modulation res'ults y.duet-o the finite attack time of the .control circuit, the audio signal to ythe modulated amplifier may be delayed slightly in order to permit the control circuit to operate before the modulation takes place.

The magnitude of the control wave for any given amount of asymmetry may be altered by' changing the turns ratio of transformer I6 or by changing the magnitude of the audio signal as supplied to Athe transformer. It is advantageous to select values which will shift the carrier level just lenough to compensate for the asymmetry, as shown in Fig. 2 at .H and I. The direction of shift may readily be changed by reversing the primary or secondary connections of transformer I6.

Referring now to Fig. 4, 4the vapplication of fthe invention to a conventional frequency-modulation ycircuit is shown. The circuit vis similar to that shown on page 242 of Hunds Frequency Modulation, first edition, published by .McGraw- Hill Book Company. Since the frequency-modulation circuit itself is Well known, detailed description of the functioning of the elements is unnecessary and it will .suffice to point out the functioning of the major components.

The audio signal input is supplied through a preemphasis circuit 6| and audio amplifier 62 to the primary of transformer 63. The signal from the secondary of transformer 63 passes through resistors 64 and 65 to the balanced modulator circuit 66. The input signal to the balanced modulator circuit may be considered to appear across terminals 61. A primary FM oscillator is shown at 68, and contains a tank circuit including inductance 69 and tunable capacitors 16 and l1.

Neglecting for the moment the asymmetry control eircuit 1|, the primary carrier frequency in tank inductance 69 is supplied through the coupled coil 'l2 and the 90 phase shifter circuit 13 to the cathode-grid circuits of reactance tubes 74 and 15. The phase-shifted carrier signal is thus supplied to both grids in the same phase. At the same time, the input audio signal at terminals B1 is supplied to the grids of reactance tubes 'I4 and .'15 in push-pull. The net effect is to vary the reactance of the tank circuit composed of inductance 69` and variable capacitors 16 and 'Il so as to obtain a frequency-modulated Wave. The output at 18 is then supplied to the usual frequency doublers and power amplifiers for transmission.

Referring now to the asymmetry control circuit 'll, an audio amplifier 8l supplies the audio signal to the balanced rectifier circuit composed of center-tapped transformer 82, rectifiers 83, B4, capacitors 85,'86, and shunting resistors lim, 90a. This lcircuit is similar to that described in connection with Fig. 3 and, as there described in detail, a control Wave appears across lines 81 whose magnitude and sign varies in .accordance with the degree and direction of asymmetry loetween positive and negative amplitudes of the audio signal Wave.

Ordinarily the balanced modulator circuit has a resistance between terminals 61. This resistance is replaced by the two resistors 88 and 89 in series with the voltage developed across capacitors 85, 86. Thus the control wave is effectively applied across terminals '61,simultaneously with the audito signals supplied thereto. Resistors 64 and 65 are inserted to prevent the shortcircuiting of the control voltage supplied to terminals '6,1 by the secondary of transformer 63, v.

f The 'operation'of the symmetry control circuit will be clear. The audio signalyoltage l'supplied to terminals 61 varies the frequencyof the modulated signal in the usual fashion. vThe .control wave, supplied to the same terminals, shifts the normal rest frequency of the transmitted signal. Normally, in the absence of any audio signal, an unmodulated rest frequency is transmitted of value determined by the constants of the tank circuit in the oscillator B9. A symmetrical audio signal varies the transmitted frequency symmetrically about the normal rest frequency. When lan asymmetrical audio signal arrives, a control voltage is developed and applied to terminals 61, thereby shifting the normal unmodulated rest carrier frequency. The asymmetrical audio signal then causes the carrier to vary around the shifted .rest frequency.

The operation is further illustrated in Fig. 5. Here, line 9| is the transfer characteristic between the input signal voltage and the frequency deviation of the modulation circuit. Line S2 is the normal unmodulated rest condition, wherein a zero input voltage gives a zero frequency deviation. If Aa symmetrical audio signal Wave 93 is applied, the symmetrical frequency deviation show-ri at 93a results. This is shown as a signal of maximum volume, so that the full frequency deviation of kc. (present standards) results. If an asymmetrical Wave such as shown at 94 is supplied to the input, and the normal rest frequency were unchanged, the frequency deviation would be as shown at 94a. This is overmodulated in the positive direction and modulated less than maximum in the negative direction.

In accordance with the invention, a contr-ol voltage of magnitude represented by Vc is suppied to the input circuit of the modulator so as to shift the normal rest carrier frequency by the amount fc. This results in shifting the asymmetrical Wave from the position shown at 94 to that shown at 95, with corresponding change in frequency deviation from 94a to 95a. The latter Wave shows that full frequency deviation is obtained from |75 kc. to -75 kc. without overmodulation.

The above specific embodiments illustrate the application of the invention to specific amplitudemodulation and frequency-modulation circuits. The application to other types of circuits will be clear to those in the art. Furthermore, the manner of deriving the control wave from the audio signal need not be the Specific one described. since many forms of peak detector circuits are known in the art and may be adapted readilyvfor use in the present invention.

It is also possible to use the principles oi the invention in audio amplifier circuits. In many amplifier circuits, particularly in class A amplifiers, the distortion introduced by the upper and lower bends of the plate characteristic is the limiting factor. If an operating point on the transfer characteristic orf the amplifier is selected to allow maximum swing for symmetrical audio signals, an asymmetrical audio signal of equal magnitude will result in distortion at either the upper or the lower bend, depending upon the direction of the asymmetry. In such case a control Wave may be developed, as above described, and supplied as a bias to the audio amplifier to shift the normal operating point on the transfer characteristic when asymmetrical signals occur. This allows maximum swing for both symmetrical and asymmetrical signals without distortion.

It will be apparent to those in the art that the principles of controlling the operation of elec- 7. tronic circuits with respect to the waveform symmetry of the input signal as described herein are of broad application. Many specic circuits may be devised or adapted to effect the desired con-- trol, within the spirit of the invention.

What is claimed is:

1. In combination, an electronic circuit having an input circuit and an output circuit with a predetermined transfer characteristic therebetween, circuit connections for applying an audio signal wave to said input circuit to swing about a predetermined operating point on said transfer characteristic, an asymmetry detection circuit supplied with said audio signal wave and designed and adapted to yield an output control wave which varies with asymmetrical positive and negative portions of said signal wave, and circuit connections for applying said control wave to said electronic circuit to shift said operating point in the direction of swing of the smaller of said asymmetrical portions when said audio signal wave is asymmetrical.

2. In combination, an electronic circuit having an input circuit and an output circuit with a predetermined transfer characteristic therebetween, circuit connections for applying an audio signal wave to said input circuit to swing about a predetermined operating point on said transfer characteristic, a rectifier circuit supplied with said audio signal wave and connected to respond to differences between positive and negative asymmetrical amplitudes thereof to produce a control wave which varies with the asymmetry, and circuit connections for applying said control wave to said electronic circuit to shift said operating point in the direction of swing of the smaller of said asymmetrical amplitudes.

3. In combination, a circuit including an electronic tube and having an input circuit and an output circuit with a predetermined transfer characteristic therebetween, circuit connections for applying an audio signal wave to said input circuit to swing about a predetermined operating point on said transfer characteristic, a balanced rectifier circuit having a plurality of peak rectiers connected to develop a control Wave proportional to the differences in amplitude of positive and negative loops of an alternating wave impressed thereon, circuit connections for impressing said audio signal wave on said rectier circuit to obtain a control wave varying with asymmetries in said signal wave, and circuit connections for applying said control wave to said electronic circuit to shift said operating point in the direction of swing of the smaller of said asymmetrical amplitudes.

4. In a modulated radio-frequency signalling system, in combination, a modulation circuit in which a selected characteristic of a radio-frequency carrier is varied in accordance with an audio signal wave, an asymmetry detection circuit supplied with said audio signal Wave and designed and adapted to yield an output control wave which varies with asymmetrical positive and negative portions of said signal wave, and circuit connections for supplying said control Wave to said modulation circuit to change the normal unmodulated value of said selected characteristic in the modulation direction of the smaller of said asymmetrical portions when said audio signal wave is asymmetrical.

5. In a modulated .radio-frequency signalling system, in combination a modulation circuit in which a selected characteristic of said radio-frequency carrier is varied from a. normal unmodulated value in accordance with said audio signal wave to produce a modulated signal, a rectier circuit supplied with said audio signal wave, said rectifier circuit being connected to respond to diiierences between positive and negative asymmetrical amplitudes of said audio signal Wave to produce a control wave which varies with the asymmetry, and circuit connections for supplying said control Wave to the modulation circuitto change said normal unmodulated value when the audio signal wave is asymmetrical, whereby overmodulation of said carrier by asymmetrical signals may be diminished.

6. In a modulated radio-frequency signalling system, in combination, a modulation circuit in which a selected characteristic of said radio-irequency carrier is varied about a normal unmodulated value in accordance with an audio signal wave to produce a modulated signal, a balanced rectier circuit having a plurality of rectiers connected to develop a control wave proportional to the dilerences in amplitude of positive and negative loops of an alternating wave impressed thereon, circuit connections for impressing said audio signal wave on said rectier circuit to obtain a control wave varying with asymmetries in said signal wave, and circuit connections for supplying said control wave to said modulation circuit to produce a proportional change in said normal unmodulated value when said audio signal Wave is asymmetrical, the polarity of the control wave as supplied to said modulation circuit being selected to change said normal unmodulated value in the modulation direction of the smaller of the asymmetrical amplitudes of said audio signal wave, whereby overmodulation of said carrier by asymmetrical signals may be diminished.

7. In an amplitude-modulation signalling system, in combination, an amplitude-modulation circuit in which the amplitude of a radio-frequency carrier is varied about a normal unmodulated level in accordance with an audio signal wave, an asymmetry detection circuit supplied with said audio signal wave and connected to respond to diiTerences between positive and negative asymmetrical amplitudes thereof to produce a control wave which varies with the asymmetry, and circuit connections for supplying said control wave to the modulation circuit to change said normal unmodulated carrier level in the modulation direction of the smaller of said asymmetrical amplitudes.

8. In an amplitude-modulation signalling system, in combination, an amplitude-modulation circuit in which the amplitude of a radio-frequency carrier is varied about a normal unmodulated level in accordance with an audio signal wave, a rectier circuit supplied with said audio signal wave and connected to respond to diierences between positive and negative asymmetrical amplitudes thereof to produce a control wave which varies with the asymmetry, and circuit connections for supplying said control wave to the modulation circuit to produce a proportional change in said normal unmodulated carrier level in the modulation direction of the smaller of said asymmetrical amplitudes.

9. In an amplitude-modulation signalling system, in combination, an amplitude-modulation circuit in which the amplitude of a radio-frequency carrier is varied about a normal unmodulated level in accordance with an audio signal wave, a balanced rectiler circuit having a plurality of peak rectiiiers connected to develop a control wave proportional to the differences in amplitude of positive and negative peaks of an alternating wave impressed thereon, circuit connections for impressing said audio signal wave on said rectifier 'circuit to obtain a control wave varying with asymmetries in said signal wave, the decay time constant of said peak rectiiiers being longerthan the period of the lowest audio frequency to' be transmitted, and circuit connections for"A supplying said control wave to said modulation circuit to change the normal unmodulated carrier level in the modulation direction of the smaller of the asymmetrical amplitudes of said audio signal wave, whereby overmodulation of said carrier by asymmetrical signals may be diminished.

l0. In `an amplitude-modulation signalling system, in combination, an amplitude-modulation circuit in which the amp-litude of a radiofrequency carrie-r is varied about a normal unmodulated level in accordance with an audio signal wave, a pair of rectifier circuits supplied with said audio signal wave, each rectier circuit containing a storage reactive impedance and the rectiers being polarized to derive rectified waves corresponding respectively to positive and negative amplitudes of the audio signal wave in respective impedances, connections for combining said rectified waves in opposition to obtain a control wave which varies with the difference in said positive and negative amplitudes when the audio signal vwave 'is asymmetrical, the reactive impedances being selected to yield a decay time constant longer than the period ofthe lowest audio frequency to be transmitted, and circuit connections for supplying said control wave to said modulation circuit to change the normal unmodulated carrier level in the modulation direction of the smaller of the asymmetrical amplitudes of said audio signal wave, whereby overmodulation of said carrier by asymmetrical signals may be diminished.

l1'."In an amplitude modulation signalling system, in combination, an amplitude-modulation circuit in which the amplitude of a radiofrequency carrier is varied abo-ut a normal unmodulated level in accordance with an audio signal wave, a.- tran-sformer supplied with said audio signal wave and having a center-tapped secondary winding, a pair of rectiers and respective capacitive irnpedances connected in circuit between said center tap and respective ends of the secondary winding and having decay time constants longer than the period of the lowest audio frequency to be transmitted, said rectiers being polarized to develop a control voltage wave across said capacitive impedances proportional to the difference in peak positive and negative amplitudes of said audio signal wave when it is asymmetrical, and circuit connections for supplying said control wave to said modulation circuit to change the normal unmodulated carrier level in the modulation direction of the smaller of the asymmetrical amplitudes of said audio signal wave, whereby overmodulation of said carrier by asymmetrical signals may be diminished.

12. In an amplitude-modulation signalling system, in combination, an amplitude-modulation circuit of the plate-modulation type including a first electronic tube having grid-cathode and plate-cathode circuits, connections for supplying a radio-frequency carrier to said gridcathode circuit, connections for supplying an audio signal wave to the plate of said electronic tube to amplitude-modulate the output thereof,

a second electronic tube in the cathode circuit of the first-mentioned electronic tube to control the plate-cathode voltage thereof and thereby the unmodulated carrier level, a transformer supplied with said audio signal wave and having a center-tapped secondary winding, a lpair of rectiflers connected in circuit between said center tap and respective ends of the secondary winding, a pair of shunt resistor-capacitor circuits in series' between said center tap vand re# spective r-ectiers and having decay time constants longer than the period of the lowest audio frequency to be transmitted, said rectiers being polarized to develop a control voltage wave across said resistor-capacitor circuits proportional to the difference in peak positive and negative amplitudes of said audio signal wave when it is asymmetrical, and connections supplying said control voltage wave to control the current tjiroufh said second electronic tube and thereby the plate-cathode voltage of the first electronic tube, the polarity of thev control wave as supplied to said second electronic tube being selectedto shift the normal unmodulated carrier level in the modulation direction of the smaller of the asyr'nmetrical amplitudes of said audio signal wave.

`13. In a frequency-modulation signalling system, in combination, a frequency-modulation circuit in which the frequency` of a radio-frequency carrier is varied about a normal unmodulated frequency in accordance with an audio signal wave, an asymmetry detection circuity sup"- plied with said audio signal vwave and connected to respond to differences between positive and negative asymmetrical amplitudes thereof to produce a control wave which varies withA the asymmetry, and circuit connections supplying said control wave to Vsaid modulation circuit to shift the normal unmodulated carrier frequency inthe modulation direction of the smaller .of said asymmetrical amplitudes. l,

14. IIn a .frequency-modulation signalling system, in combination, a freouency-modulation circuit in whichthefrequencyof a Yradio-freguency carrier is variedabout a normal unmodulated frequency inuac'cordance with van audio signal wave. a rectifier circuit supplied with said audio signal Wave and connected torespond to differences between positive and negative asymmetrical amplitudes thereof to produce a control wave which varies with the asymmetry, and eircuit connections supplying said control wave to Said llflOdlllatll Circuit t() DIdllC a prlltilll shift in the normal unmodulated carrier frequency in the modulation direction of the smaller of said asymmetrical amplitudes.

l5. In a frequency-modulation signalling system, in combination, a frequency-modulation circuit in which the frequency of a radio-frequency carrier is varied about a normal unmodulated frequency in accordance with an audio signal wave, a balanced rectifier circuit having a plurality of peak rectiflers connected to develop a control wave proportional to the differences in amplitude of positive and negative peaks of an alternating wave impressed thereon, circuit connections for impressing Said audio signal wave on said rectifier circuit to obtain a control wave varying with asymmetries in said signal wave, the decay time constant of said peak rectiiiers being longer than the period of the lowest audio frequency to be transmitted, and circuit connections supplying said control wave to said modulation circuit to shift the normal unmodulated t carrier frequency in the modulation direction of the smaller of the asymmetrical amplitudes of said audio signal wave, whereby overmodulation of said carrier by asymmetrical signals may be diminished.

16. In a frequency-modulation signalling system, in combination, a frequency-modulation circuit in which the frequency of a radio-frequency carrier is varied about a normal unmodulated frequency in accordance with an audio signal wave, a pair of peak rectiier circuits supplied with said audio signal wave, each rectifier circuit containing a capacitive impedance and the rectiers being polarized to derive rectified Waves corresponding respectively to positive and negative amplitudes of the audio signal wave in respective impedances, connections for combining said rectified waves in opposition. to obtain a control wave which varies with the difference in said positive and negative amplitudes when the audio signal wave is asymmetrical, the capacitive impedances being selected to yield a decay time constant longer than the period of the lowest audio frequency to be transmitted, and circuit connections supplying said control wave to said modulation circuit to shift the normal unmodulated carrier frequency inthe modulation direc' tion of the smaller of the asymmetrical amplitudes of said audio signal wave, whereby overmodulation of said carrier by asymmetrical signals may be diminished.

1'7. In a frequency-modulation signalling system, in combination, a frequency-modulation circuit having an input circuit supplied with an audio signal wave to swing the frequency of a radio-frequency carrier about a normal unmodulated Value, a transformer supplied with said audio signal wave and having a center-tapped secondary winding, a pair of rectiflers and respective capacitive impedances connected in circuit between said center tap and respective ends .zia

asymmetrical amplitudes of the audio signal wave.

18. In an electronic circuit having a predetermined transfer characteristic between input and output circuits thereof, and an audio signal wave applied to said input circuit to swing about a predetermined operating point on said transfer characteristic, the method which comprises developing a control wave which varies with asymmetrical positive and negative portions vof said audio signal wave, and utilizing said control wave to shift said operating point in the direction of swing of the smaller of said asymmetrical portions 'when said audio signal wave is asymmetrical.

19. In an electronic circuit having a predetermined transfer characteristic between input and output circuits thereof, and an audio signal wave applied to sai-d input circuit to swing about a predetermined operating point on said transfer characteristic, the method which comprises developing a control wave which is proportional to the difference between positive and negative asymmetrical amplitudes of said audio signal wave, and utilizing said control wave to shift said operating point in proportion thereto and in the direction of swing of the smaller of said asymmetrical amplitudes.

20. In a modulated radio-frequency signalling system in which a selected characteristic of a radio-frequency carrier is modulated in accordance with an audio signal wave, the method which comprises developing a control wave which is proportional to the difference between positive and negative asymmetrical amplitudes of said audio signal wave, and shifting the normal unmodulated value of said selected characteristic in accordance with said control wave and in the modulation direction of the smaller of said asymmetrical amplitudes, whereby overmodulation of said carrier by asymmetrical signals may be diminished.

LUKE H. MONTGOMERY.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,114,336 Ditcham Apr. 19, 1938 2,285,896 Brown June 9, 1942 

